HD Advisor November 6, 2009

[Attebery]

He hasn't come down from his castle just yet, but when he does, you can discuss the latest installment of High-Def Digest's very own HD Advisor right here!

[mak99]

Q: I recently re-did my family room into a multi-purpose media room, and I've come across an issue that I cannot seem to solve. When I am listening to anything, be it audio or film, when I am adjusting the volume I get feedback. I have an Onkyo TX-SR 875 and when adjusting the volume I get a "bump" sound through the sub-woofer, on every .5 increment. So imagine the volume rapidly increasing or decreasing, there is a rapid thump. Now I'm not sure if this is worth mentioning, but I ran dedicated power (20 amp circuit) for the Onkyo thinking that I would not get issues like this, but alas it was not to be. All cables were custom made and buzzed to ensure there would be no shorts in the cables. My speaker system consists of Athena Technologies (an API Company) S3 and P3 Series of speakers. Not sure if you are familiar with these speakers but they are modular. The S3 and P3 can be attached (via Ni-Cad railings) and run in full range, or be separated and run as, well, separates. Which means that I am running 2 subs. Now the Onkyo only having the one sub output makes me wonder if splitting it may in fact be the issue. I have tried bypassing cables, running the cables away from line voltage cables, I just cannot solve this and I hope you will be able to offer a solution.

We sorta have a similar setup - Onkyo TX-SR805 with two Polk microPro subs connected to the single sub out w/ a Y-connector. I have no issues like you are having, whatsoever. To be specific, my subs are stand-alone types, not part of a connected system like your Athenas, which I'm not familiar with.

I do have all my AV gear connected thru a Panamax M5410Pro conditioner. This is plugged into a wall outlet, which this 15A circuit does have the overhead lights included. (FWIW, I will eventually run a dedicated 20A circuit just for the AV system as there is a spare available in the main panel.)

You might try plugging your Onkyo into another outlet to verify it's not your 20-amp circuit, which I doubt is the cause. Can you hook up other speakers and a "regular" sub to see if you have the same bumping issue?

[Gunyaga]

I'm thinking this is most likely a loop in the way those speakers configure causing the feedback. Given the rest of the electrical setup, I'd start there. Not saying that has to be it by any means, just that I think that is the most likely culprit.

I agree with the suggestion to try hooking up to a separate line level amplified sub and see of the problem persists, if it does not, that points to the speaker setup.

[Attebery]

He's baaaaaack:

http://www.highdefdigest.com/news/sh...sor_35_Up/3700

[Drazden]

Okay... the input is much appreciated. I never thought of plugging into a different outlet to see if that fixes the problem.
I do have a different setup I can try. A pair of Boston Acoustics bookshelf speakers and a Sub. will try that and see if there is a difference.
As for the setup. The subs are powered subs and are not connected by the Ni-Cad railings. they are actually set up independently as you would normally setup your standard sub.
If it is a grounding loop with the 20 Amp circuit, thats any easy fix. if its the actual setup, :-(, thats not gonna make me happy... I'll keep my fingers crossed...

Again, Thanks for the advice

[Josh Z]

A reader emailed this:

---
In response to your homework question, I am inclined to believe that you need to have some better cables to connect the subs (and maybe other coaxial devices) to prevent the ground loop problem that I believe you have. Ordinary coaxial cables are two conductor. The center conductor carries the signal and the shield is the return and the ground. In a complex system, this will not work well because interconnected components, even if they all have 3-wire power, do not have the same chassis potentials. This forces current to flow through the shield wires and also can cause hum and the reception of rf (radio) signals along with the switching noise your receiver is making when you adjust the volume.

You need to have shielded pair cables with the shield connected to only ONE end of the cable. Normally this is the receiver end. Monster used to make cables like this (perhaps they still do) The put a stripe on the connector to designate the end where the shield is connected to help the user install them properly. In the past, I have made my own shielded-pair cables that worked very well. Long cable runs aggravate the problems of ground loops. I don't know your specific situation, but I am nearly certain that this is your problem.